Conventionally, an image forming apparatus cannot be used if a sensor for detecting printing paper is out of order and the paper being conveyed cannot be detected normally. The image forming apparatus of which the sensor is out of order cannot be used until the sensor out of order is repaired or exchanged. Generally, the job of repairing or exchanging the sensor is carried out by a service man armed with professional knowledge. That is, if the sensor is out of order, there is a problem that the conventional image forming apparatus cannot be used until the sensor out of order is repaired or exchanged by a service man.